Traditional Principles of Animation/Overlapping action
Overlapping action implies that not everything happens at the same time.
Take the example of moving a character's arm from a position of rest to that of picking up a set of keys on a table. A common mistake would be to advance a few frames from the at-rest position, then move all of the arm elements (objects) to the final position. This technique would result in a very lifeless motion (because everything starts and stops moving at the same time). The proper sequence requires that the upper arm begins to rise first. Then the forearm pivots out, followed by the wrist bending back. Finally the fingers curl around the keys. Each of these motions begins before the preceding motion is complete, providing the realistic overlap that your viewer expects. In other words, the motion of the arm rotating upward is overlapped by the wrist bending back.